1. Field of the Invention
This invention is in the field of coating of substrates like pharmaceutical tablets, and it is specifically concerned with coating such substrates with a coating that acts as a moisture barrier.
2. Description of the Prior Art
Many medicinal substances and the dosage forms into which they are made undergo degradation during storage due to contact with moisture which has been absorbed from the atmosphere. To combat this degradation, it is frequently necessary for such moisture sensitive dosage forms to be packed in special containers to limit the exposure of the dosage forms to the atmosphere.
In territories where the climate is very humid, specialized packaging does not provide a totally satisfactory answer to moisture degradation.
Elimination of the need for the special packaging, or augmentation of the moisture protection provided by such packaging, may be achieved in the case of solid dosage forms, such as tablets, capsules and granules, by coating with materials which reduce the rate at which the dosage form absorbs atmospheric moisture. Moisture barrier coatings of this type have been employed previously. The traditional means of providing the moisture barrier characteristics required for moisture barrier coatings has been by the use of polymers which are soluble in organic solvents or are relatively insoluble in water. The disadvantages of this approach are 1) the need to use either organic solvents or difficult aqueous polymer dispersion techniques to apply these water-insoluble polymer materials, and 2) the reduced Aqueous solubility of the coating leading to prolonged disintegration time of the dosage form in the body when ingested, with consequent delayed or reduced activity of the medicinal substance in the tablet.
The use of the polymer polyvinyl alcohol, PVA, as a moisture barrier coating has been previously suggested. However, practical usage has been inhibited by the stickiness of grades of the polymer which have a fast enough rate of going into solution in water to make a dispersion to render them economical to use in making the coating. A further problem with the use of PVA is in identifying or selecting a plasticizer which does not compromise the moisture barrier properties of the final coating.